Megatsunami
A megatsunami is an extremely large wave created by a substantial and sudden displacement of material into a body of water.
Megatsunamis have different features from ordinary tsunamis. Ordinary tsunamis are caused by underwater tectonic activity and therefore occur along plate boundaries and as a result of earthquakes and the subsequent rise or fall in the sea floor that displaces a volume of water. Ordinary tsunamis exhibit shallow waves in the deep waters of the open ocean that increase dramatically in height upon approaching land to a maximum run-up height of around in the cases of the most powerful earthquakes. By contrast, megatsunamis occur when a large amount of material suddenly falls into water or anywhere near water. They can have extremely large initial wave heights in the hundreds of metres, far beyond the height of any ordinary tsunami. These giant wave heights occur because the water is "splashed" upwards and outwards by the displacement.
Examples of modern megatsunamis include the one associated with the 1883 eruption of Krakatoa, the 1958 Lituya Bay earthquake and megatsunami, and the 1963 Vajont Dam landslide. Prehistoric examples include the Storegga Slide, and the Chicxulub, Chesapeake Bay, and Eltanin meteor impacts.
Overview
A megatsunami is a tsunami with an initial wave amplitude measured in many tens or hundreds of metres. The term "megatsunami" has been defined by media and has no precise definition, although it is commonly taken to refer to tsunamis over high. A megatsunami is a separate class of event from an ordinary tsunami and is caused by different physical mechanisms.Normal tsunamis result from displacement of the sea floor due to movements in the Earth's crust. Powerful earthquakes may cause the sea floor to displace vertically on the order of tens of metres, which in turn displaces the water column above and leads to the formation of a tsunami. Ordinary tsunamis have a small wave height offshore and generally pass unnoticed at sea, forming only a slight swell on the order of above the normal sea surface. In deep water it is possible that a tsunami could pass beneath a ship without the crew of the vessel noticing. As it approaches land, the wave height of an ordinary tsunami increases dramatically as the sea floor slopes upward and the base of the wave pushes the water column above it upwards. Ordinary tsunamis, even those associated with the most powerful strike-slip earthquakes, typically do not reach heights in excess of.
By contrast, megatsunamis are caused by landslides and massive earthquakes that displace large volumes of water, resulting in waves that may exceed the height of an ordinary tsunami by tens or even hundreds of metres. Underwater earthquakes or volcanic eruptions do not normally generate megatsunamis, but landslides next to bodies of water resulting from earthquakes or volcanic eruptions can, since they cause a much larger amount of water displacement. If the landslide or impact occurs in a limited body of water, as happened in Lituya Bay and at the Vajont Dam, then the water may be unable to disperse and one or more exceedingly large waves may result.
Submarine landslides can pose a significant hazard when they cause a tsunami. Although a variety of different types of landslides can cause tsunami, all the resulting tsunami have similar features such as large run-ups close to the tsunami, but quicker attenuation compared to tsunami caused by earthquakes. An example of this was the 17 July 1998 Papua New Guinean landslide tsunami, in which waves up to high struck a section of the coast, killing 2,200 people, yet at greater distances the tsunami was not a major hazard. This is due to the comparatively small source area of most landslide tsunami which causes the generation of waves with shorter wavelengths. These waves are greatly affected by coastal amplification and radial damping.
The size of landslide-generated tsunamis depends both on the geological details of the landslide and also on assumptions about the hydrodynamics of the model used to simulate tsunami generation, thus they have a large margin of uncertainty. Generally, landslide-induced tsunamis decay more quickly with distance than earthquake-induced tsunamis, as the former, often having a dipole structure at the source, tend to spread out radially and have a shorter wavelength while the latter disperses little as it propagates away perpendicularly to the source fault. Testing whether a given tsunami model is correct is complicated by the rarity of giant collapses.
Recent findings show that the nature of a tsunami depends upon the volume, velocity, initial acceleration, length, and thickness of the landslide generating it. Volume and initial acceleration are the key factors which determine whether a landslide will form a tsunami. A sudden deceleration of the landslide may also result in larger waves. The length of the slide influences both the wavelength and the maximum wave height. Travel time or run-out distance of the slide also will influence the resulting tsunami wavelength. In most cases, submarine landslides are noticeably subcritical, that is, the Froude number is significantly less than one. This suggests that the tsunami will move away from the wave-generating slide, preventing the buildup of the wave. Failures in shallow waters tend to produce larger tsunamis because the wave is more critical as the speed of propagation is less. Furthermore, shallower waters are generally closer to the coast, meaning that there is less radial damping by the time the tsunami reaches the shore. Conversely tsunamis triggered by earthquakes are more critical when the seabed displacement occurs in the deep ocean, as the first wave has a shorter wavelength and is enlarged when travelling from deeper to shallower waters.
Determining a height range typical of megatsunamis is a complex and scientifically debated topic. This complexity is increased by the two different heights often reported for tsunamis – the height of the wave itself in open water and the height to which it surges when it encounters land. Depending upon the locale, this second height, the "run-up height," can be several times larger than the wave's height just before it reaches shore. While there is no minimum or average height classification for megatsunamis that the scientific community broadly accepts, the limited number of observed megatsunami events in recent history have all had run-up heights that exceeded. The megatsunami in Spirit Lake in Washington in the United States generated by the 1980 eruption of Mount St. Helens reached, while the tallest megatsunami ever recorded reached a run-up height of. It is also possible that much larger megatsunamis occurred in prehistory; researchers analyzing the geological structures left behind by prehistoric asteroid impacts have suggested that these events could have resulted in megatsunamis that exceeded in height.
Recognition of the concept of megatsunami
Before the 1950s, scientists had theorized that tsunamis orders of magnitude larger than those observed with earthquakes could have occurred as a result of ancient geological processes, but no concrete evidence of the existence of these "monster waves" had yet been gathered. Geologists searching for oil in Alaska in 1953 observed that in Lituya Bay, mature tree growth did not extend to the shoreline as it did in many other bays in the region. Rather, there was a band of younger trees closer to the shore. Forestry workers, glaciologists, and geographers call the boundary between these bands a trim line. Trees just above the trim line showed severe scarring on their seaward side, while those from below the trim line did not. This indicated that a large force had impacted all of the elder trees above the trim line, and presumably had killed off all the trees below it. Based on this evidence, the scientists hypothesized that there had been an unusually large wave or waves in the deep inlet. Because this is a recently deglaciated fjord with steep slopes and crossed by a major fault, one possibility was that this wave was a landslide-generated tsunami.On 9 July 1958, a 7.8 strike-slip earthquake in Southeast Alaska caused of rock and ice to drop into the deep water at the head of Lituya Bay. The block fell almost vertically and hit the water with sufficient force to create a wave that surged up the opposite side of the head of the bay to a height of, and was still many tens of metres high further down the bay when it carried eyewitnesses Howard Ulrich and his son Howard Jr. over the trees in their fishing boat. They were washed back into the bay and both survived.
Analysis of mechanism
The mechanism giving rise to megatsunamis was analysed for the Lituya Bay event in a study presented at the Tsunami Society in 1999; this model was considerably developed and modified by a second study in 2010.Although the earthquake which caused the megatsunami was considered very energetic, it was determined that it could not have been the sole contributor based on the measured height of the wave. Neither water drainage from a lake, nor a landslide, nor the force of the earthquake itself were sufficient to create a megatsunami of the size observed, although all of these may have been contributing factors.
Instead, the megatsunami was caused by a combination of events in quick succession. The primary event occurred in the form of a large and sudden impulsive impact when about 40 million cubic yards of rock several hundred metres above the bay was fractured by the earthquake, and fell "practically as a monolithic unit" down the almost-vertical slope and into the bay. The rockfall also caused air to be "dragged along" due to viscosity effects, which added to the volume of displacement, and further impacted the sediment on the floor of the bay, creating a large crater. The study concluded that:
A 2010 model that examined the amount of infill on the floor of the bay, which was many times larger than that of the rockfall alone, and also the energy and height of the waves, and the accounts given by eyewitnesses, concluded that there had been a "dual slide" involving a rockfall, which also triggered a release of 5 to 10 times its volume of sediment trapped by the adjacent Lituya Glacier, as an almost immediate and many times larger second slide, a ratio comparable with other events where this "dual slide" effect is known to have happened.